Hydraulic control system for construction machine

Abstract

Provided is a hydraulic control system for a construction machine, including: a control valve ( 31 ) for controlling supply and discharge of hydraulic fluid to and from an arm cylinder ( 4 ); an operation lever ( 6 ) for controlling the position of a spool of the control valve ( 31 ); a meter-out flow passage ( 34 ) for passing therethrough hydraulic fluid discharged from the arm cylinder; a variable restrictor ( 23 a ) provided in the meter-out flow passage; pressure sensors ( 41, 42 ) for detecting a magnitude of a negative load applied by an external force to the arm cylinder in a same direction as an actuation direction of the arm cylinder; and a controller ( 45 ) for reducing an opening area of the variable restrictor ( 23 a ) according to an increase in the magnitude of the negative load calculated with a detection value from the pressure sensors ( 41, 42 ).

The invention claimed is: 
     
       1. A construction machine, comprising:
 a hydraulic actuator driven by hydraulic fluid delivered from a hydraulic pump; 
 a control valve configured to control supply and discharge of hydraulic fluid to and from the hydraulic actuator according to a position of a spool; 
 an operation device configured to control the position of the spool of the control valve according to an operation amount and an operation direction; 
 one or plurality of meter-out flow passages disposed to pass therethrough hydraulic fluid discharged from the hydraulic actuator; 
 at least one variable restrictor provided in the one meter-out flow passage, or at least one variable restrictor provided in each of the plurality of meter-out flow passages; 
 a load sensor configured to detect a magnitude of a negative load applied by an external force to the hydraulic actuator in a same direction as an actuation direction of the hydraulic actuator; and 
 a control device configured to reduce, according to an increase in the magnitude of the negative load detected by the load sensor, an opening area of the one variable restrictor in a case where only one variable restrictor is provided, or a sum of the opening areas of the plurality of variable restrictors in a case where two or more variable restrictors are provided, 
 wherein a range in which the opening area of the one variable restrictor or the sum of the opening areas of the plurality of variable restrictors is varied by the control device according to the increase in the magnitude of the negative load detected by the load sensor has an upper limit value and a lower limit value with respect to each value of the operation amount of the operation device, and 
 wherein the upper limit value and the lower limit value are increased according to an increase in the operation amount of the operation device. 
 
     
     
       2. The construction machine according to  claim 1 , wherein:
 the one meter-out flow passage is a first flow passage through which hydraulic fluid discharged from the hydraulic actuator flows when the hydraulic actuator is actuated in the same direction as the negative load, the first flow passage passing through the control valve, 
 the at least one variable restrictor is a first variable restrictor provided in the control valve in the first flow passage, and 
 the control device is configured to reduce the opening area of the first variable restrictor by changing the position of the spool of the control valve according to the increase in the magnitude of the negative load detected by the load sensor. 
 
     
     
       3. The construction machine according to  claim 1 , wherein:
 the plurality of meter-out flow passages include:
 a first flow passage through which hydraulic fluid discharged from the hydraulic actuator flows when the hydraulic actuator is actuated in the same direction as the negative load, the first flow passage passing through the control valve, and 
 a second flow passage through which hydraulic fluid discharged from the hydraulic actuator flows when the hydraulic actuator is actuated in the same direction as the negative load, the second flow passage being branched from a middle of the first flow passage, 
 
 the at least one variable restrictor provided in each of the plurality of meter-out flow passages includes:
 a first variable restrictor provided in the control valve in the first flow passage and having an opening area that increases according to an increase in the operation amount of the operation device, and 
 a second variable restricting device provided in the second flow passage and having an opening area that increases according to an increase in a pilot pressure output from a hydraulic fluid source, and 
 
 the control device is configured to reduce the opening area of the second variable restrictor according to the increase in the magnitude of the negative load detected by the load sensor, and thereby reduces the sum of the opening areas of the first variable restrictor and the second variable restrictor according to the increase in the magnitude of the negative load detected by the load sensor. 
 
     
     
       4. The construction machine according to  claim 3 , wherein:
 the hydraulic fluid source of the pilot pressure for the second variable restricting device is a pilot pump or the operation device that allows hydraulic fluid from the pilot pump to be depressurized and output.